The present invention relates to an electric lamp having a ceramic discharge vessel enclosing a discharge space having a length L, a diameter D, and an aspect ratio L/D; a fill gas including xenon, mercury, sodium halide, and halides of rare earth metals; and a pair of electrodes for maintaining a discharge in the fill gas.
High wattage (over 150 W) metal halide lamps are presently available only with quartz discharge vessels, which are larger than ceramic vessels and have a lower (xe2x88x92200xc2x0 C.) wall temperature. A smaller vessel is desirable because the smaller discharge vessel better approximates a point source. Higher temperatures are desirable to achieve a higher cold spot temperature TC on the vessel wall; this increases the vapor pressure of the salts in the fill gas. The term xe2x80x9cceramicxe2x80x9d as used herein means metal oxide, such as sapphire or polycrystalline alumina (PCA), as well as nitrides such as AlN.
U.S. Pat. No. 5,973,453 discloses a ceramic discharge metal halide (CDM) lamp wherein EA/D greater than 5, EA being the distance between electrode tips, the tips being spaced from the endwalls of the discharge space. The ionizable filling includes Xe as an ignition gas, and NaI and CeI3 in a molar ratio between 3:1 and 7:1. In an embodiment having a rated power of 150 W and intended as a retrofit for a high pressure sodium installation operating at 80-100 volts, EA/D=8, the fill includes Hg, and the Xe fill pressure is 250 mbar (187 torr). This yields a color rendering index (CRI) of 58 at a color temperature of 3900xc2x0 K., and a luminous efficacy of 130 lm/W. It is noted that a comparable HPS lamp has a lower luminous efficacy (110 lm/W) and considerably lower CRI (21), while a comparable high pressure mercury lamp has comparable CRI but much lower efficacy (60 lm/W).
In another 150 W embodiment disclosed in U.S. Pat. No. 5,973,453, the fill is free of mercury and the lamp is operated at 45 V, so it is not suited as a retrofit for HPS. The Xe fill pressure is 1250 mbar (938 torr), the efficacy is 145 lm/W, and the CRI is 53. In a 185 W embodiment, the lamp voltage is 53 V, the Xe fill pressure is 500 mbar (375 torr), and the CRI is 61. All embodiments use a ceramic tube with a wall thickness of 1.4 mm. All Hg-free embodiments are operated on a square wave voltage generated by an electronic ballast.
While U.S. Pat. No. 5,973,453 discloses a CDM lamp with high efficacy, and even suggests a possible retrofit for an HPS ballast, the color rendering is still less than desirable and would not be suitable for many applications.
U.S. Pat. No. 6,031,332 discloses a CDM lamp having a CRI over 90, and achieves a limited voltage crest factor, so that the lamp achieves a long useful life. Voltage crest factor VCF is the ratio of the reignition voltage to the arc voltage, i.e. the operating voltage. The reignition voltage is the voltage required to reignite the discharge when it extinguishes as the polarity of an AC supply voltage changes. VCF assumes a high value in particular when the lamp is operated on a sinusoidal voltage, which is typical of a magnetic ballast, and usually increases during lamp life.
U.S. Pat. No. 6,031,332 addresses the problem of increasing reignition voltage by including calcium iodide in the fill to a molar quantity of 30 to 50% of the total molar quantity of halides. The ratio EA/D is less than 1.0 and L/D is slightly greater than 1.0; the fill includes argon at a pressure of 140 mbar (105 torr) as the ignition gas. The lamp operates at 80 to 100 V but the power is only 70 W; as such it would not be suitable for retrofit in an HPS installation.
A well known problem in metal halide lamps is the occurrence of hydrogen iodide voltage spikes. HI spikes occur during run-up of metal halide lamps that have hydrogen contamination in the presence of free iodide. Typically, the hydrogen comes from water that is present in the fill gas, but it can also be present on the lamp parts and the salts. Special precautions are required to insure that the H2O level inside the arc tube is as low as possible, preferably less than 0.5% of the fill gas.
One method to eliminate the HI spikes is given in U.S. Pat. No. 4,409,517, which discloses the use of Nb as a window to allow the rapid diffusion of H2 out of the arc tube. U.S. Pat. No. 4,203,049 discloses a getter for the hydrogen.
The prior art does not disclose a high wattage CDM lamp with good color rendering, high efficacy, and high lumen maintenance which would be suitable for use with an existing magnetic ballast for an HPS lamp.
It is a primary object of the invention to provide a high wattage (over 150 W) CDM lamp which can be used with a magnetic ballast which was designed for use with a high pressure sodium (HPS) lamp. It is a related object to provide a CDM lamp which limits hydrogen iodide voltage spikes so they are within the voltage supplied by the ballast.
It is a further object to provide a CDM lamp which has a low voltage crest factor so that flicker is eliminated and long life is achieved using an HPS ballast.
It is a further object to determine a design space for the lamp that is within established material limits while providing the desired lamp efficacy and color properties.
These and other objects are achieved in a CDM lamp using xenon as a starting gas and having an aspect ratio in the range of 3 to 5. This is considered a medium aspect ratio, since most prior art CDM lamps have aspect ratios of about 1, and HPS lamps have aspect ratios on the order of 10.
The design space was determined by the use of designed experiments and the characteristic equations for each design parameter. FIG. 1 shows the design space that was found for a 200 W CDM lamp. Four curves were plotted. The curve on the lower left represents a voltage crest factor VCF of 1.7, and the space above it represents lower VCF""s. This is desirable because ballasts for HPS lamps have low sustaining voltages. The next curve represents a wall temperature TW of 1250xc2x0 C., and the space above it represents lower wall temperatures. This is desirable because at higher temperatures the PCA is attacked by the salts and also evaporates, which darkens the outer envelope and shortens lamp life. Next are two intersecting curves which define the actual design space. One is the curve for a cold spot temperature of 1005xc2x0 C.; the space above it represents lower temperatures. The other represents an efficacy of 90 lm/W; the space below it represents higher efficacies. The design space is limited by an inside diameter of 6.7 mm and an inside length of 33 mm (aspect ratio 4.9), and, at an inside diameter of 8 mm, lengths of 25 mm (aspect ratio 3.1) and 30 mm (aspect ratio 3.8). Longer lengths may be possible.
The design space for the discharge tube is also limited by the need to reduce or eliminate hydrogen iodide voltage spikes. It was found that the level of HI spike voltage during the first run-up of the lamp is dependent on both the volume V of the arc tube and the cold fill pressure P of the starting gas (also called buffer or ignition gas). In a series of experiments with Xe as the gas, the minimum HI spike voltage was measured and plotted against the product of P and the volume, as shown in FIG. 4. A curve fit to the data is described by the equation VHI=33654 PVxe2x88x921.185, where P is in torr and V is in cubic centimeters. To minimize H2 and H2O in these experiments, the arc tubes were made in an inert gas atmosphere dry box, the discharge tube was vacuum baked at 1300xc2x0 for one hour, the electrodes were vacuum baked at elevated temperature, and the salts were contained in an inert gas atmosphere until dosed into the arc tube. In spite of these careful steps, HI spikes still form, but can be controlled by choice of P and V.
For a lamp to sustain on a ballast, the voltage spike must be below the available voltage supplied by the ballast. This voltage is typically 200 volts or more for lamps whose nominal lamp voltages are over 90 volts. The spike voltage should be less than 180 volts and practically less than 150 volts and preferably less than 100 volts for reliable starting. Plugging these voltages into the fitted equation, or reading the plot of FIG. 4, yields the following results: PV=82.7 torr-cc for 180 volt spike; PV=96.4 torr-cc for 150 volt spike; PV=135.8 torr-cc for 100 volt spike.
The design space for the discharge tube is further limited by the need to limit the voltage crest factor VCF. It has been found that VCF of a new CDM lamp follows a curve that is inversely proportional to the product of the total pressure PTOT and the square of the inner diameter, as shown in FIG. 5. The equation for the curve is VCF=39616/PTOTD2+1.359. In order for the lamp to run on existing HPS or other types of ballasts, to prevent flicker, and to promote long lamp life, VCF should be less than 1.7. Lamps will achieve a VCF of less than 1.7 if PTOT D2 greater than 1.16xc3x97105 torr-mm2. The total pressure can be calculated from the Hg dose and arc tube volume. The arc tube volume is computed for a cylinder having a diameter D and a length L using the formula V=xcfx80LD2/4, a well known method for computing the volume of a cylinder. Assuming a parabolic temperature profile, the total pressure is PTOT=748 Hg/V+8.87 Pxe, where 748 has the units cm3-torr/mg, Hg is the dose in mg, V is in cm3, and Pxe is in torr. The last two equations can be designed to get a requirement for low VCF in terms of construction parameters:
9.524xc3x97105 Hg/L+8.87D2Pxe greater than 1.16xc3x97105.
The data points in FIG. 5 are from lamps operated on CWA ballasts. The total pressures were calculated from known Hg doses, Xe fill pressures, and arc tube volumes.
The advantages of the CDM lamp according to the invention are that it provides a high efficacy (over 90 lm/W), white light (xcx9c4000xc2x0 K. CCT, MPCD+/xe2x88x9210), and a high CRI (over 85) in a 200 W lamp. CCT is the correlated color temperature and MPCD is the minimum perceptible color difference, a measure of the color point from the black body line. The lamp also exhibits color stability and lamp-to-lamp color uniformity previously only enjoyed by lower wattage CDM lamps such as Mastercolor lamps (Mastercolor is a registered trademark of Philips Electronics North America Corporation). Additionally, the lamp is suitable as a retrofit for 200 W HPS S-66 ballasts.